Development of an apparatus which evaluates structure of magnetic domains on a magnetic material, is performed in parallel with density growth in magnetic recording to the magnetic material which is a recording medium. The scanning tunneling microscopy and the scanning electron microscopy using spin-polarized electrons are expected to have a resolution equal to or less than 5 nm. However, these apparatuses can only perform observation of the surface of magnetic material which is extremely clean, and it is not easy to apply them as practicable evaluation apparatuses or inspection apparatuses provided in a manufacturing line. Consequently, it is suggested to use the magnetic force microscopy (hereinafter referred to as “MFM”) which can observe the magnetic domain structure on an insulating protective film, as an evaluation apparatus for the magnetic domain structure. In the MFM, detected is a force which affects a probe of a cantilever having a magnetic material or a current path and which is caused by a leak magnetic field from a sample. However, when the measurement distance between the surface of the sample and the probe is too small, since affection of the van der Waals force between the surface of the sample and the probe becomes large and quantitative observation of the magnetic domain structure becomes difficult, it is absolutely necessary in the quantitative observation to separate the probe from the surface of the sample by a distance larger than or equal to a predetermined distance. As the result, spatial resolution stays lower than or equal to 10 nm in the present situation.
On the other hand, in “Using a magnetometer to image a two-dimensional current distribution”, by Bradley J. Roth et al., Journal of Applied Physics, United States, American Institute of Physics, Jan. 1, 1989, Vol. 65, No. 1, p. 361-372 (Document 1), proposed is the technique that the relationship between electric current and a magnetic field is mathematized with use of the Biot-Savart law in the experiment where flux change is measured by using the superconducting quantum interference device, and a current density distribution is calculated from the magnetic field measured at the position above the surface of the sample. In addition, a possibility of obtaining information of magnetized state in positions including a surface of thin film and a cross section of the thin film by the MFM, is mentioned in Japanese Patent Application Laid-Open No. 2002-257705 (Document 2), and Japanese Patent Application Laid-Open No. 2002-366537 (Document 3) discloses a method of alternately and iteratively performing a amendment which satisfies the Dirichlet condition and a amendment which satisfies the Neumann condition, on an approximate solution of the Laplace equation when a potential problem satisfying the Laplace equation and having a mixing boundary value at a boundary is solved.
In the meantime, the technique proposed in the Document 1 premises that the current density distribution exists only on the surface of the sample, and the technique can not be used as a common tool to analyze a magnetic field.